Design an approach for the sidebar in the deprecations app

[mansona]

This is blocked by ember-learn/empress-blog-ember-template#23

We need to first come up with a solution for sidebars in each of our apps and then once that is done we need to implement a sidebar specifically for this app based on the design system.

Note: this is only blocked assuming that the blog is "going first" which is currently under discussion 👍